Refactor LT/GT into a oneof

[elliotmjackson]

relates to #77

In the given changes to the file "proto/protovalidate/buf/validate/validate.proto", several modifications have been made to the message definitions. The following actions were taken:

• Previously existing oneof fields "_gt", "_gte", "_lt", and "_lte" were removed from messages like "FloatRules," "DoubleRules," "Int32Rules," and so on.
• Fields within these messages were restructured: fields originally associated with "_lt" and "_lte" were moved to the "less_than" oneof, and fields originally associated with "_gt" and "_gte" were moved to the "greater_than" oneof.
• Furthermore, certain fields originally associated with "_lt_now" were moved to the "less_than" oneof, and fields associated with "_gt_now" were moved to the "greater_than" oneof.